1. Technical Field
This invention relates to solid-state lasers, and more particularly to an active mirror amplifier laser having a side-pumped gain medium disposed in contact with an actively cooled substrate.
2. Background of the Invention
In solid-state lasers (SSL), optical pumping generates a large amount of heat within a laser medium and increases its temperature. Continuous operation of the laser, therefore, requires removal of the waste heat by cooling selected surfaces of the laser medium. Because SSL media typically have a low thermal conductivity, a significant thermal gradient is created between the hot interior and the cooled outer surfaces. This causes a gradient in the index of refraction, mechanical stresses, depolarization, detuning, and other effects, with likely consequences of degraded beam quality, reduced laser power, and possibly a fracture of the SSL medium. Such effects present a major challenge to scaling of SSLs to high-average power (HAP). Pumping by semiconductor laser diodes, which was introduced in the last decade, greatly reduces the amount of waste heat and paves the way for development of a HAP-SSL with good beam quality. Such lasers are expected to make practical new industrial processes such as precision laser machining with applications ranging from deep penetration welding to processing of aerospace materials.
It has been long recognized that optical distortions caused by transverse temperature gradients (i.e., perpendicular to laser beam axis) degrade beam quality. A class of SSL known as xe2x80x9cactive mirror amplifierxe2x80x9d (AMA) originally disclosed by Almasi et al. in U.S. Pat. No. 3,631,362 (1971) has shown effective reduction of transverse temperature gradients and demonstrated the generation of a laser output with very good beam quality. See, for example, J. Abate et al., xe2x80x9cActive Mirror: A large-aperture Medium Repetition Rate Nd: Glass Amplifier,xe2x80x9d Appl. Opt. Vol. 20, no. 2, 351-361 (1981) and D. C. Brown et al., xe2x80x9cActive-mirror Amplifier: Progress and Prospects,xe2x80x9d IEEE J. of Quant. Electr., vol. 17, no. 9, 1755-1765 (1981).
In the classical AMA concept, a large aspect ratio, edge-suspended, Nd-Glass disk (or slab) several centimeters thick is pumped by flashlamps and liquid-cooled on the back face. However, this device is not suitable for operation at HAP because of poor heat removal and resulting thermo-mechanical distortion of the edge-suspended disk. Previous attempts to mitigate these problems and increase the average power output of an AMA were met with encouraging but limited results. In recent years, the AMA concept has been a revived in the form of a xe2x80x9cthin disk laserxe2x80x9d introduced by Brauch et al. in U.S. Pat. No. 5,553,088. The thin disk laser uses a gain medium disk which is several millimeters in diameter and 200-400 xcexcm in thickness soldered to a heat sink. See, for example, A. Giesen et al., xe2x80x9cScalable concept for diode-pumped high-power lasers,xe2x80x9d Appl. Phys. B vol. 58, 365-372 (1994). The diode-pumped Yb:YAG thin disk laser has demonstrated laser outputs approaching 1 kW average power and with beam quality around 12 times the diffraction limit. See, for example, C. Stewen et al., xe2x80x9c1-kW CW Thin Disk Laser,xe2x80x9d IEEE J. of Selected Topics in Quant. Electr., vol. 6, no. 4, 650-657 (July/August 2000). Another variant of the thin disk laser can be found in L. Zapata et al., xe2x80x9cComposite Thin-Disk Laser Scalable To 100 kW Average Power Output and Beyond,xe2x80x9d in Technical Digest from the Solid-State and Diode Laser Technology Review held in Albuquerque, N.Mex., Jun. 5-8, 2000.
The applicant""s patent application Ser. No. 99/505,399 titled Active Mirror Amplifier System and Method for a High-Average Power Laser System, hereby incorporated by reference, discloses a new AMA concept, which is suitable for operation at high-average power. The invention uses a large aperture laser gain medium disk about 2.5 mm in thickness and with a diameter typically between 5 cm and 15 cm mounted on a rigid, cooled substrate. Note that the disk thickness in this AMA concept is about 10 times less than in the classical AMA and about 10 times more than in the thin disk laser. The substrate contains a heat exchanger and microchannels on the surface facing the laser medium disk. The disk is attached to the substrate by a hydrostatic pressure differential between the surrounding atmosphere and the gas or liquid medium in the microchannels. This novel approach permits thermal expansion of the laser medium disk in the transverse direction while maintaining a thermally loaded disk in a flat condition. The teachings of this patent application provide numerous advantages over prior art SSL and allow generation of near diffraction limited laser output at very high-average power from a relatively compact device.
The above-mentioned patent application Ser. No. 99/505,399 also teaches two principal methods for providing pump radiation into the AMA disk, namely 1) through the large (front or back) face of the disk, or 2) through the sides (edges) of the disk. The former method is often referred to as xe2x80x9cface pumpingxe2x80x9d and is further elaborated on in J. Vetrovec, xe2x80x9cDiode-pumped Active Mirror Amplifier For High-Average Power,xe2x80x9d in proc. from Lasers 2000 Conference held in Albuquerque, N.Mex., Dec. 4-8, 2000. This publication describes a face-pumped AMA with pump radiation from a diode array injected into the laser gain medium through an optically transparent substrate.
To make face pumping efficient, the AMA disk must absorb a large fraction of the pump radiation injected. This condition can be met by a certain combination of disk thickness and doping density of lasant ions. However, in many cases of interest it is impractical (or undesirable) to make the necessary increase in disk thickness or lasant doping level. For example, doping a yttrium-aluminum garnet (YAG) crystal with neodymium (Nd3+) ions beyond about 1.5% of atomic concentration is known to reduce the fluorescence time, broaden the line-width, and excessively stress the crystal due to a mismatch in size between the Nd atoms and yttrium atoms (the latter being replaced in crystal lattice). Increasing the disk thickness is often undesirable as it also increases thermal impedance and leads to higher thermal stresses. These considerations limit design parameters of face-pumped AMA to a relatively narrow regime. Face pumping is also impractical in conjunction with ytterbium (Yb3+) lasant ions, which require very high pump intensities to overcome re-absorption of laser radiation by the ground energy state. For example, a 2.5 mm-thick AMA disk made of YAG crystal would require about 10% atomic doping concentration of Yb3+ ions to absorb 90% of face-injected pump radiation in two passes. Such a high Yb concentration would require an unreasonably high pump intensity of about 34 kW/cm2 to induce medium transparency at 1.03 xcexcm wavelength, and several times this level to efficiently operate the laser. In this situation, injecting the pump radiation into the disk side (i.e., edge or perimeter) becomes an attractive alternative. Side-pumping takes advantage of the long absorption path (approximately same dimension as the diameter of the gain medium disk), which permits doping the disk with a reduced concentration of lasant ions. This in turn reduces requirements for pump radiation intensity.
While side-pumping may be a suitable method for delivering pump radiation, several associated technical challenges still need to be overcome, such as: 1) delivering and concentrating pump radiation into the relatively small area around the disk perimeter; 2) preventing overheating of the disk in the areas where the pump radiation is injected; 3) generating uniform laser gain over the AMA aperture; and 4) avoiding laser gain depletion by amplified spontaneous emission (ASE) and parasitic oscillations. The significance of these challenges and related solutions disclosed in the prior art are discussed below.
1. Concentration of Pump Radiation
Modern SSL are optically pumped by semiconductor lasers commonly known as laser diodes. Because each laser diode produces a relatively small optical output (up to a few watts), pumping of SSL for HAP requires the combined output of a great many laser diodes (typically in quantities ranging from hundreds to hundreds of thousands). For this purpose the diodes are arranged in one-dimensional arrays often called xe2x80x9cbarsxe2x80x9d containing about 10 to 20 diodes and two-dimensional arrays often called xe2x80x9cstacksxe2x80x9d containing several hundred diodes. Stacks are typically produced by stacking about 10-20 bars and mounting them onto a heat exchanger. A good example of commercially available stacks is the Model SDL-3233-MD available from SDL, Inc., of San Jose, Calif., which can produce 200 xcexcs-long optical pulses with a total output of 960 watts at a maximum 20% duty factor. SSL for HAP may require a combined power of multiple units of this type to produce desired pumping effect in the laser gain medium. Regardless of the grouping configuration, individual laser diodes emit optical radiation from a surface, which is about 1 xcexcm high and on the order of 100 xcexcm wide. As a result, the beamlet of radiation emitted from this surface is highly asymmetric: highly divergent in a direction of the 1 xcexcm dimension (so called xe2x80x9cfast axisxe2x80x9d) and moderately divergent in the transverse dimension (so-called xe2x80x9cslow axisxe2x80x9d). This situation is illustrated in FIG. 2. Typical fast axis divergence angles (full-width at half-maximum intensity) range from 30 to 60 degrees, while slow axis divergence angles typically range from 8 to 12 degrees. Optical radiation from an array of diodes has similar properties. High divergence in the fast axis makes it more challenging to harness the emitted power of diode arrays for use in many applications of practical interest. Some manufacturers incorporate microlenses in their laser diode arrays to reduce fast axis divergence to as little as a few degrees. An example of such a product is the lensed diode array Model LAR23P500 available from Industrial Microphotonics Company in St. Charles, Mo., which includes microlenses which reduce fast axis divergence to less than three degrees.
The intensity of the optical output of diode arrays (lensed or unlensed) is frequently insufficient to pump a SSL gain medium to inversion, and the radiation must therefore be further concentrated. In previously developed systems, optical trains with multiple reflecting and/or refracting elements have been used. See, for example, F. Daiminger et al., xe2x80x9cHigh-power Laser Diodes, Laser Diode Modules And Their Applications,xe2x80x9d SPIE volume 3682, pages 13-23, 1998. Another approach disclosed by Beach et al., in U.S. Pat. No. 5,307,430 uses a lensing duct generally configured as a tapered rod of rectangular cross-section made of a material optically transparent at laser pump wavelength. Operation of this device relies on the combined effect of lensing at the curved input surface and channeling by total internal reflection. Light is concentrated as it travels from the larger area input end of the duct to the smaller area exit end. Yet another approach for concentrating pump radiation disclosed by Beach et al. in U.S. Pat. No. 6,160,939 uses a combination of a lens and a hollow tapered duct with highly reflective internal surfaces.
2. Thermal Control of Disk Perimeter
The surfaces of the laser gain medium that receive pump radiation are susceptible to overheating and, as a result, to excessive thermal stresses. Experience with end-pumped rod lasers shows that a composite rod having a section of doped and undoped laser material provides improved thermal control and concomitant reduction in thermal stresses. See, for example, R. J. Beach et al., xe2x80x9cHigh-Average Power Diode-pumped Yb:YAG Lasers,xe2x80x9d UCRL-JC-133848 available from the Technical Information Department of the Lawrence Livermore National Laboratory, U.S. Department of Energy. A suitable method for constructing composite optical materials of many different configurations is disclosed by Meissner in U.S. Pat. No. 5,846,634.
3. Uniform Laser Gain Across the Aperture
Due to the exponential absorption of pump radiation, portions of the laser gain medium that are closer to the pump source are susceptible to being pumped more intensely than portions that are further away. Non-uniform deposition of pump energy results in non-uniform gain. Gain non-uniformities across the laser beam aperture (normal to the laser beam axis) are highly undesirable as they lead to degradation of beam quality. In prior art devices, non-uniform pump absorption has been compensated for in a side-pumped rod laser by the gain medium being fabricated with a radially varying level of doping. An alternate approach known as xe2x80x9cbleach-wave pumpingxe2x80x9d has been proposed by W. Krupke in xe2x80x9cGround-state Depleted Solid-state Lasers: Principles, Characteristics and Scaling,xe2x80x9d Opt. and Quant. Electronics, vol. 22, S1-S22 (1990). Bleach wave pumping largely depletes the atoms in the ground energy state and pumps them into higher energy states. Achieving high uniformity of gain becomes even more challenging as the incident laser beam causes saturation-induced change in the spatial distribution of gain. Thus, the weaker portions of the signal are amplified relatively more than the stronger portions because they saturate the medium to a lesser degree.
4. Amplified Spontaneous Emission (ASE)
Amplified Spontaneous Emission (ASE) is a phenomenon wherein spontaneously emitted photons traverse the laser gain medium and are amplified before they exit the gain medium. The favorable condition for ASE is a combination of high gain and a long path for the spontaneously emitted photons. ASE depopulates the upper energy level in an excited laser gain medium and robs the laser of its power. Furthermore, reflection of ASE photons at gain medium boundaries may provide feedback for parasitic oscillations that aggravate the loss of laser power. If unchecked, ASE may become large enough to deplete the upper level inversion in high-gain laser amplifiers. Experimental data suggests that in q-switched rod amplifiers ASE loss becomes significant when the product of gain and length becomes larger than 2.25, and parasitic oscillation loss becomes significant when the product is larger than 3.69. See, for example, N. P. Barnes et al., xe2x80x9cAmplified Spontaneous Emissionxe2x80x94Application to Nd:YAG Lasers,xe2x80x9d IEEE J. of Quant. Electr., vol. 35, no. 1 (January 2000). Continuous wave (CW) or quasi-CW lasers are less susceptible to ASE losses because their upper level population (and hence their gain) is clamped.
A traditional method for controlling ASE losses to an acceptable level is disclosed, for example, by Powell et al. in U.S. Pat. No. 4,849,036. This method involves cladding selected surfaces of the laser gain medium with a material that can efficiently absorb ASE radiation. To reduce the reflection of ASE rays at the cladding junction, the cladding material must have an index of refraction at the laser wavelength that is closely matched to that of the laser gain medium. Recently, another method for ASE loss control was introduced. In this method, ASE rays are channeled out of selected laser gain medium surfaces into a trap from which they are prevented from returning. See, for example, R. J. Beach et al., xe2x80x9cHigh-average Power Diode-pumped Yb:YAG Lasers,xe2x80x9d supra.
In view of the foregoing limitations with previously developed AMAs, it is an object of the present invention to provide an active mirror amplifier (AMA) capable of operating at high-average power and good beam quality (BQ). In particular, the AMA of the present invention meets a number of significant needs:
a side-pumped AMA for a HAP;
means to avoid excessive losses to ASE and parasitic oscillations;
means for trapping ASE rays and significantly reducing feedback to parasitic oscillations;
means for concentrating pump radiation for injection into the AMA disk side;
means for concentrating pump radiation by a circular arrangement of pump sources;
means for alleviating thermal stresses and reducing the temperature near surfaces of the laser gain medium where pump radiation is injected;
means for controlling the gain profile across the AMA aperture;
means for efficient operation of an AMA-HAP with quasi-3 level laser media such as Yb3+;
means for efficient operation of an AMA-HAP with laser media exhibiting high pump saturation intensities;
an AMA with laser diode pump means that reduces the waste heat load to the solid-state laser medium;
a relatively thin solid-state medium to allow efficient conduction of waste heat;
microchannel cooling of a support substrate for efficient removal of waste heat from the laser gain medium;
a substrate which provides rigid mechanical support for the solid-state laser medium;
the use of concentrator ducts for the delivery of pump power to the sides (edges) of the AMA laser gain medium;
a composite gain medium assembly for the delivery of pump radiation, reduced thermal distortions and reduced ASE/parasitic losses;
hydrostatic pressure means to maintain the solid-state gain medium in an optically flat condition on said substrate;
attachment means that reduce thermally-induced distortions in the solid-state gain medium;
pre-forming laser gain medium to reduce thermally-induced stresses therein;
a longer absorption path for pump radiation, which allows reducing the laser gain medium doping requirements and concomitant reabsorption losses for the gain media of 3-level lasers (e.g., Yb:YAG); and
a means for pressure-balanced coolant fluid transfer to cool the laser gain medium.
The AMA of the present invention can be used as a building block for construction of laser oscillators as well as laser amplifiers. In one preferred embodiment the invention comprises a laser gain medium having a front surface, a rear surface and a peripheral edge. The rear surface is attached to a cooled support substrate. One or more sources of optical pump radiation are disposed so as to inject optical pump radiation into one or more sections of the peripheral edge of the gain medium. Optionally, an undoped optical medium may be attached to the peripheral edge of the laser gain medium inbetween the peripheral edge and one of the sources of optical pump radiation. Alternatively, the undoped optical medium may cover the entire peripheral edge of the laser gain medium.
In one preferred embodiment a plurality of hollow, tapered ducts are arranged inbetween the peripheral edge of the laser gain medium and the sources of optical pump radiation. The hollow, tapered ducts help to direct or xe2x80x9cchannelxe2x80x9d optical pump radiation in the peripheral edge of the laser gain medium. The sources of optical pump radiation are comprised of pluralities of laser diode arrays arranged to direct optical pump radiation through the hollow, tapered ducts.
The precise shape of the laser gain medium may vary considerably, with circular, elliptical rectangular, hexagonal, octagonal and polygonal shapes all being possible. The undoped optical medium may form one or a plurality of sections circumscribing the peripheral edge of the laser gain medium. The section(s) may be secured to the peripheral edge via an optically transparent bond. Various preferred embodiments of the arrangement of the optical pump sources and the laser gain medium are disclosed.